CT scans allow an image of the internal structure of a target object to be generated, one cross-sectional slice at a time. Typically, the target object is an anatomical region of a patient, although CT systems can also be used in non-medical applications, for example explosive detection. In a CT system, x-rays emitted from an x-ray source are passed through a region of the object, then are detected by a detector assembly. The detector assembly, consisting of one or more rows of detector elements, generates detection signals indicative of the attenuated intensities of the x-rays that have traversed the object. The detection signals are sent to a computer, which implements signal and image processing techniques to reconstruct a tomographic image of the object.
In a helical CT scanner, the patient is translated (typically at a constant speed), while the x-ray source and the detector assembly rotate around the patient. As the patient is moving, the data for the prescribed number of axial slices of the target region (within the patient) is acquired. Because the patient table is translated at a constant speed along the axis of rotation of the gantry during helical scanning, the image location, also called the slice plane, constantly moves in the axial direction. The trajectory of the source relative to the slice plane maps out a helix, generating projection data from which axial image slices may be reconstructed. Helical scanners offer a number of advantages, including reduced scanning time, improved image quality, and better control of contrast.
In a multi-slice helical scanner, more than one row of detector elements are arranged side-by-side along the rotation axis of the scanner. For a given view angle, it is thus possible to obtain projections that measure attenuation in multiple sections of the scanned object in a single sampling period. Multi-slice scanners are becoming the norm for medical CT applications. Multi-slice scanners make rapid acquisition of volumetric data possible, because of the larger coverage that they provide (compared to the coverage provided by single slice scanners), coupled with helical scanning. However, since in helical multi-slice scanners the patient is translated for each sample, the projections measure attenuation at different positions within the patient. This necessitates extracting consistent data sets for each cross-sectional slice position from the helical multi-slice data, in order to reconstruct each cross-sectional slice, thereby adding further complications to the image reconstruction process.
Currently, volumetric reconstruction for multi-slice scanners is typically performed by helical interpolation, followed by 2D filtered back-projection. Generally, the number of rows in multi-slice scanners is limited to about four, in order to prevent image artifacts. When the number of rows is limited to four, it has been found that helical interpolation ignoring the divergence of the x-ray beam, coupled with 2-D backprojection, is sufficient to provide clinically acceptable image quality. In this case, an interpolation filter is used to estimate virtual fan beam data for an image at a desired plane, given the positions of the plane, the source and the detector. For a larger number of rows, it has been observed that 3-D backprojection is necessary to provide acceptable image quality. Asymmetric fan beams have been used to increase a scanner's field of view (FOV) in a cost effective manner, because the FOV can be increased by increasing the number of detectors on only one side of the fan beam. When the FOV is increased in this manner, resolution is lowered in the extended part of the FOV.
All of the features described above, namely helical scanning, multiple detector rows and asymmetric fan beam, are desirable features for a CT scanner (or PET scanner), because of the advantages described above. In order to implement such an asymmetric, multi-slice, helical scanner, an appropriate image reconstruction method and system is required.
Accordingly, it is an object of the present invention to provide a method and system for CT image reconstruction, that can be used with an asymmetric multi-slice helical scanner, and that produces acceptable image quality.